1. Field of the Invention
The present invention relates to an electric device for medium and high voltage transmission and/or distribution lines containing as insulating filler a silicone-based composition having improved chemical-physical characteristics, in particular, dielectric strength and compressibility.
2. Description of the Related Art
It is known that electric devices for medium and high voltage transmission and/or distribution lines, such as circuit breakers, insulated switch-gears, power and distribution transformers, instrument transformers, surge arresters, cable terminations, pole heads, bushings, insulators and similar, generally contain suitable free volumes filled with an insulating filler. In particular, said insulating filler, which can be solid or semisolid, liquid or gaseous, must basically guarantee a desired dielectric strength among the parts of the device having different voltage due to the normal electrical functioning.
At the present state of the art, use of well known insulating fillers for different applications and/or specific needs presents some drawbacks.
For example, when liquid insulating fillers are used, typically dielectric oils, compensating volumes must be employed to counteract effects caused by thermal expansions of insulating liquids; in the case of gaseous insulating fillers, such as sulphur hexafluoride (SF.sub.6) or nitrogen and/or their mixtures, systems monitoring the gas pressure as well as gas filling up devices are necessary to keep insulating properties unchanged.
In addition, in both case of gaseous and liquid insulating fillers, special devices and security systems must be used to prevent and/or to detect filler leakages which could cause misfunctioning of the electric device as well as environmental pollution.
This affects the electric devices in terms of structural complexity and also overall reliability of the insulating system.
In some applications, solid or semisolid materials, such as polyurethane, silicone foams or rubbers, are also used as insulating fillers. In particular, such solid fillers give the advantage to prevent leakages from the electric device as well as to eliminate special devices and security systems; but, on the other hand, they present the main problem of requiring accurate process controls to reach a good quality of adhesion to the surrounding parts of the device. As a matter of fact, in the case of defective and non-homogeneous adhesion to the electric device surrounding parts, inceptions of destructive electric discharges may happen due to air filtering; these electric discharges could cause the electric device breakdown.
In order to avoid this drawback, adhesion promoters have to be used and all surfaces must be treated with primers so as to obtain a good and homogeneous filler adhesion; such treatments are expensive and complicated especially in the case of electric devices having complex geometry and in particular when functional elements such as cables, mechanical rods, connections under voltage are present.
Another problem related to the use of known solid or semisolid insulating fillers is due to the intrinsic properties of such materials generally presenting high coefficients of thermal expansion associated to a negligible, like a liquid, compressibility; this fact represents a limit to the applications.
For instance, the use of cross-linked silicone elastomers for electric applications as insulating fillers, is already known. These materials have a thermal expansion coefficient of about 10.sup.3 C.sup.-1 and a low compressibility, comparable to the typical value of a liquid; since in these applications the silicon elastomer is inserted in closed spaces and expands because of a possible heating, particurarly in the case of high-voltage applications, damages to the electric device could happen.